1. Field of Invention
This invention relates to portland and other hydraulic cements, and more particularly to a process and apparatus for the making of such cements.
2. Description of the Prior Art
The hydraulic cements represent an important group of cementing materials which are used principally in the construction industry. These cements have the special property of setting and hardening under water. The essential components of the cements are lime (CaO), silica (SiO.sub.2) and components derived from them. In the presence of water, these components react to form, ultimately, a hardened product containing hydrated calcium silicate. The hydraulic cements include portland cement as well as high alumina cement, hydraulic lime, and other lesser known cements.
Of all the hydraulic cements, portland cement is by far the most important, for this cement is a major construction material that is utilized in practically all concrete as well as in most of the masonry mortars. The principal components of portland cement are tricalcium silicate (3CaO.SiO.sub.2), dicalcium silicate (2CaO.SiO.sub.2), and tricalcium aluminate (3CaO.Al.sub.2 O.sub.3), all of which, when in a ground or powdered condition, will react with water to form a hard, stone-like substance held together with intermeshed crystals. Other compounds, such as magnesium oxide (MgO) and tetracalcium aluminoferrite (4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3), which are present in portland cement, do not exhibit any cementitious properties. The exact composition of portland cement is defined in A.S.T.M. Standard Specifications which are accepted by the industry.
Generally speaking, portland cement is currently obtained by finely intergrinding lime and silica containing materials and heating the mixture within a rotary kiln to the point of fusion. Fusion occurs at or about 1290.degree. C., the precise temperature depending upon the chemical composition of the feed materials and the type and amount of fluxes that are present in the mixture. The principal fluxes are alumina (Al.sub.2 O.sub.3) and iron oxide (Fe.sub.2 O.sub.3), and these fluxes enable the chemical reactions to occur at relatively lower temperatures. Normally the lime is obtained from natural calcareous deposits such as limestone, marl, and aragonite. Under certain conditions, lime may be derived from industrial by-products such as phospho-gypsum, a pluverulent calcium sulfate which may be obtained from the manufacture of phosphoric acid. The silica and fluxes, on the other hand, are normally derived from natural argillaceous deposits such as clay, shale, and sand.
More specifically, to manufacture portland cement, an argillaceous material and a calcareous material are crushed, mixed, and interground to a find powder, with the proportions of the two materials and the composition of each being maintained within narrow limits. The mixing and intergrinding may be done in the dry condition (the dry process) or it may be done in water (wet process). In either case, the mixture passes into the upper end of a rotary kiln where it is heated eventually to the fusion point. However, before this point, water and carbon dioxide are driven off. As the hottest region is approached, a part of the interground mixture of materials melts and chemical reactions take place between the constituents of the raw mixture. In the course of these reactions new compounds are formed. After passing the hottest region, the compounds fuse and form a clinker. The clinker then is discharged into some form of a cooler. When cool, the clinker is mixed with a carefully controlled quantity of gypsum, and the mixture is ground to a very fine powder. That finely ground powder is the portland cement of commerce.
Rotary kilns vary in length and diameter. They revolve slowly (one turn in every 1 to 2 minutes or more) and, as they are slightly inclined, the charge slowly travels downwardly toward the hot end of the kiln. Being heated from its lower end, a rotary kiln develops its hottest temperatures in a rather narrow zone of the kiln, with the temperature becoming progressively less toward the upper end. At no time does the entire mixture in the rotary kiln, even in the hottest zone, become molten. Special refractories are required, especially for the hot zone at the lower end, and once the kiln is fired it must remain in operation, lest the expensive refractory will be destroyed upon cooling. Attempting to operate a rotary kiln above its normal operating temperature range will result in a high percentage of the feed mixture becoming liquid at one time and running uncontrollably out of the kiln. It will also cause severe damage to the refractories and to the kiln shell.
Generally, a rotary kiln is heated by burning a fossil fuel at its lower end, with the hot combustion gases traveling up the kiln. Heat energy is transferred to the downwardly moving raw feed by direct contact and indirectly by heating the refractory lining. As the raw materials become dried, heated, and partly calcined by the hot gases, some of the finer particles are picked up and transported out of the kiln as kiln dust.
The kiln dust usually contains some alkalies, primarily in the form of compounds of sodium and potassium, for they are usually found in the raw feed and also in coal which is used as a fuel. Also the raw feed and fuel often contain sulfur which volatilizes and enters the gas stream where it usually combines with lime and alkalies for form sulfates. The kiln dust is usually returned to the kiln, but eventually its alkali or sulfate level becomes so great that it is not suitable for manufacturing cement and must be discarded. This presents a disposal problem. Those sulfur compounds that do not combine with alkalies or lime leave with the flue gases. If sulfur is sufficiently high in quantity, the flue gas stream may become environmentally unacceptable and require treatment to meet emission standards.
In short, the present kiln process for manufacturing portland cement requires a large capital investment, and consumes an enormous amount of fuel. Furthermore, the kiln must remain heated, once it is fired, since the thermal shock encountered upon cooling will destroy its refractory which is quite expensive in itself.